1. Field of the Invention
The present invention relates to an electronic balance for measuring a weight of an article including a Roberval""s mechanism for keeping a balance pan in horizontal and a lever mechanism as a lever for transmitting a displacement of a height of a balance pan to a load sensor, which are integrated into one body by hollowing out a base material of the electronic balance.
2. Description of the Related Art
JP-B-6-29761 disclosed an example of an electronic balance of the related art. In a related art, an aluminum alloy body like a parallelepiped is hollowed out from a side thereof so that the aluminum alloy body is formed into a predetermined shape. In this way, a mechanism body of the electronic balance including a detecting part has an integral structure in which the hollowed aluminum alloy body and elastic parts are integrally formed into one body. The structure is shown in FIGS. 5 and 6. A mechanism body 1 is fixed to a base frame 2 by screws 3. Sheet-shaped Roberval""s parts 4, 5 are respectively formed in an upper and a lower end portion of the mechanism body 1. A lever 6 elastically supported by a contracted part 6a, which becomes a fulcrum, is formed below the Roberval""s part 4. A force coil 7 is fixed to an end portion of the lever 6. The other end portion of the lever 6 is connected to a movable part 10 by a connecting part 9. The contracted parts are formed at both ends of the connecting part 9.
A receiving rod 11 is fastened to an upper end portion of the movable part 10. A balance pan 12 for measurement is fixed to the receiving rod 11. A periphery of the force coil 7 is hollowed out. A magnet 8 is fixed to the hollowed portion of the force coil 7. Although not shown in the drawing, at the end portion of the lever 6 corresponding to the force coil 7, there is provided a zero position sensor for detecting the displacement using a photo sensor. In this type electronic balance, a control circuit is provided except for the aforementioned components. (not shown)
In measuring a weight, an object to be measured is put on the balance pan 12. When the weight of the object is given, the movable part 10 is displaced downward while the balance pan 12 keeping in horizontal position because the upper and the lower face of the movable part 10 are supported by the Roberval""s parts 4, 5. This downward displacement of the movable part 10 is transmitted to an end portion of the lever 6 by the connecting part 9, and then the end portion of the lever 6 on the force coil 7 side is displaced upward as a fulcrum of the contracted part 6a. 
This displacement of the end portion of the lever 6 is detected by the zero position sensor. An electronic current flows in the force coil 7 by the control circuit so that the above displacement can become zero by an attraction force generated by the force coil 7 and the magnet 8. Accordingly, the electronic current proportional to the load of the object put on the balance pan 12, flows in the force coil 7. At this time, an intensity of the electronic current flowing in the force coil 7 is converted into a weight and displayed on a display part, which is not shown in the drawing.
The electronic balance is composed as described above. Therefore, it is necessary to conduct machining such as screw thread cutting for connecting the base frame 2 to the mechanism body 1. It is also necessary to fasten the receiving rod 11 to the mechanism body 1. When the screws 3 are fastened, as shown in FIG. 6, stress strain parts 3a, 11a are generated in the periphery of a screw thread cutting part 14 of the mechanism body 1. By an influence of this stress strain given to the stress strain parts 3a, 11a, an error is caused when the object to be measured is biased and put on the balance pan 12.
In order to solve the problem of the above error caused when the object to be measured is put on the balance pan being biased, a distance from the Roberval""s parts, 4, 5 to the screw thread cutting part 14 may be sufficiently long so that the measurement cannot be affected by the screw thread cutting part 14.
In order to avoid the influence of the measurement error caused by the stress strain, there is provided another method in which a slit is between the Roberval""s parts 4, 5 and the screw thread cutting part 14.
However, when this method is adopted, the mechanism body 1 is extended in a longitudinal direction.
(Hereinafter, the longitudinal direction is defined as a direction from the receiving rod 11 to a joining portion, at which the mechanism body 1 is connected to the base frame 2). Further, it is necessary to optimize a shape of the slit and a position at which the slit is formed. Therefore, it is necessary to apply a complicated technique.
Accordingly, in any of the above methods, the mechanism body 1 is extended in the longitudinal direction. In general, in the case of an electronic balance, the balance pan using for measurement is set at the center of the balance. Therefore, when a size only in one direction is extended, it becomes difficult to downsize the electronic balance. The present invention has been accomplished in view of the above problems.
It is an object of the present invention to provide an electronic balance being capable of solving a measurement error, which is caused by stress strain in a joining portion of the electronic balance, without extending the size of the electronic balance in a longitudinal direction when an object to be measured is put being biased.
(Hereinafter, the longitudinal direction is defined as a direction from a receiving rod to a joining portion, at which a mechanism body is connected to a base frame.)
In order to solve the above problem, an electronic balance comprising:
a balance pan for putting an article thereon;
a movable part fixed to the balance pan; and
a Roberval""s mechanism for moving the movable part in parallel in the vertical direction,
wherein the movable part, Roberval""s mechanism, and each connecting part of the movable part and Roberval""s mechanism are formed into an integral construction,
a lever mechanism for transmitting a displacement of the movable part to a load sensor;
a display device for displaying a weight of the article detected by the load sensor;
a base frame for fixing the integral construction; and
a protruding portion formed at least in a joining portion where the integral construction is fixed to the base frame and in an attached portion where the balance pan is attached in the integral construction, a width of the protruding portion being larger than that of the Roberval""s mechanism,
wherein at least one of the balance pan and the base frame is joined at the protruding portion.
From a first aspect of the invention, a distance from the joining portion to the Roberval""s mechanism can be made sufficiently long. The electronic balance of the invention has a structure in such a manner that the Roberval""s mechanism is not affected by stress strain, which is caused when the screw is fastened, without extending the size of the electronic balance in the longitudinal direction.
As a result, it becomes possible to solve the problem of an error caused when an object to be measured is put being biased. It also becomes possible to reduce the size of the electronic balance in the longitudinal direction. Therefore, a compact electronic balance can be put into practical use.